You can set the default scanner to be used for SOHODOX from the Options dialog.

 To do this, 

1. Click the round button on the top left corner of the screen. 
2. Select Options... the Options dialog will be launched In this dialog.
3. Scroll down to find Default Scan Device:,
4. Select the default scan device you want and click OK. 

The default scan device will now be set.

You can also use Scan Profiles incase you have multiple scanners on the network and use a different settings for each of them. 

Scan Profile is a collections of frequently used settings which can be saved and reused to scan a document. So instead of specifying these settings every time you scan, you can specify these settings once and save them as a Scan Profile and choose the profile whenever you scan a document from a particular scanner. 

More info on Scan Profiles is available here...

http://www.sohodox.com/docs/help/index.htm?save_the_frequently_used_scan_settings.htm